Method of producing tar sands with laterally cratered nuclear explosions

ABSTRACT

Two or more nuclear explosives are detonated a predetermined distance below a tar sand in a manner such that one explosion craters laterally into a cavity formed by an earlier explosion to form a relatively thin unique zone of rubble below the tar sand. The rubble is composed of material other than tar sand. The predetermined distance below the tar sand does not exceed 250 feet or three cavity radii whichever is smaller. Fluids are injected into this rubble to assist in producing oil from the tar sands.

United States Patent Inventor Appl. No.

Filed Patented Assignee Henry F. Dunlap Dallas, Tex.

Sept. 25, 1969 Dec. 14, 1971 Atlantic Richfield Company New York, N.Y.

Continuation-impart of application Ser. No. 628,143, Apr. 3, 1967, now Patent No. 3,470,953. This application Sept. 25, 1969, Ser. No. 861,132

METHOD OF PRODUCING TAR SANDS WITH LATERALLY CRATERED NUCLEAR EXPLOSIONS 22 Claims, 5 Drawing Figs.

US. Cl

Int. Cl

[50] Field of Search 166/247, 256, 259, 271, 272, 299, 302, 303

[56] References Cited UNITED STATES PATENTS 3,233,670 2/1966 Thompson et al. 166/247 3,303,881 2/1967 Dixon 166/247 3,464,490 9/1969 Silverman 166/247 X 3,470,953 10/1969 Dunlap 166/247 Primary Examiner-Stephen J. Novosad Attorneys-Blucher S. Tharp and David Folzenlogen ABSTRACT: Two or more nuclear explosives are detonated a predetermined distance below a tar sand in a manner such that one explosion craters laterally into a cavity formed by an earlier explosion to form a relatively thin unique zone of rubble below the tar sand. The rubble is composed of material other than tar sand The predetermined distance below the tar sand does not exceed 250 feet or three cavity radii whichever is smaller. Fluids are injected into this rubble to assist in produc ing oil from the tar sands.

Patented Dec. 14, 1971 3,627,044

2 Sheets-Sheet 1 Fig. 1

INVENTOR Henry F. Dunlap BY qMJW ATTORNEY Patented Dec. 14, 1971 3,627,044

2 Sheets-Shoot 2 gllllll Fig. 4

INVENTOR Henry F. Dunlap BY 7 W rd/$060110)! ATTORNEY METHOD OF PRODUCHNG TAR SANDS Wl'illl-ll LATERALILY QRATEIRED NUCLEAR EXPLOSHONS.

CROSS-REFERENCE TO RELATED APPLICATION This application is a continuation-in-part of copending application, Ser. No. 628,l43, filed Apr. 3, 1967, now U.S. Pat. No. 3,470,953, by the same inventor as this application and owned by a common assignee.

BACKGROUND OF THE INVENTION This invention relates to a method of assisting in production of oil from tar sands using multiple nuclear explosives. More particularly, nuclear explosives are detonated in a special manner to form a unique relatively thin zone of rubble below a tar sand and fluids are used to produce oil from the tar sand.

This invention involves production of oil from tar or highly viscous oil sands. Such sands contain a bituminous material which is thick, adhesive and sticky and will change its form under the influence of a deforming force. As will hereinafter be shown, this invention solves a problem created by these peculiar characteristics of tar sands.

When a nuclear explosive of proper yield is detonated in or below a tar sand, high amounts of energy are produced in microseconds and an initial cavity is formed as a result of vaporization, melting and crushing of adjacent media. The expanding energy of the gases compacts and thrusts the surrounding media outward, creating in fractions of a second an unstable spherical cavity. The cavity expands until the internal pressure is about equivalent to the over burden pressure. At this point the gas pressure supports the overburden, thus preserving the shape of the cavity for a temporary period of time. The radius of the cavity is a function of the energy yield of the explosive, and, to a much lesser extent, the rock media characteristics and depth of burial.

After a period of time, the pressure is reduced and, the fractured media above the cavity is no longer supported. The roof of the cavity collapses in bits and pieces which periodically fall to the bottom of the cavity. A cylindrical column of broken tar sand or rock develops upward as the cavity fills. Roof collapse continues progressively upward until the volume or interstitial space between the pieces of fallen broken formation approximates the original cavity volume before the cavity began to collapse. The vertical, cylindrical volume of broken formation, called a chimney, has roughly the same diameter as the original cavity and has a height of about four or five times the cavity radius. The ratio of chimney height to original cavity radius is, therefore, dependent on the bulk porosity of the chimney and the cavity volume before collapse. More explicitly, the height of the chimney is about four times the cavity radius divided by three times the net increase in porosity in the rubbled zone stated in a fraction. This fraction ranges between 0.2 and 0.3 and for petroleum reservoirs will probably be on the order of 0.25.

It has been proposed to use nuclear explosives to assist in producing oil from tar sands. The nuclear explosive is placed in or below the tar sand and is of sufficiently low yield that the explosion will not break through the surface of the earth. Since tar sands are relatively shallow and the ordinary chimney formed by a nuclear explosion has a height several times its diameter, the yield of the explosive is seriously limited. The effects of this limitation have been substantially overcome by the sequence ofsteps mentioned in copending application Ser. No. 628,143, now US. Pat. No. 3,470,953, which is incorporated herein.

It has also been proposed to use an array of nuclear explosives to create lateral masses of broken bituminous formations. Insofar as these proposals relate to laterally extending masses of broken formation, the disclosures teach that multiple shots could be fired in two ways. The shots could be fired simultaneously or substantially simultaneously. When two shots are fired simultaneously, each shot occurs before a cavity is completed by another shot. As an alternative, laterally spaced shots could be fired in sequence. This would be done by drilling an emplacement hole and loading and firing each hole before drilling the next hole. When two shots are fired in this manner, the second shot is fired after a chimney is formed by the first shot. The second laterally spaced explosion, if properly spaced from the first shot, creates fractures in the media between the two shots and there is little or no change in individual chimney geometry. in addition, the flow channels in the rubble in the chimneys are undesirably larger than the channels in the fractured area between two chimneys, and the interconnecting fractures created by such explosions are not uniform and continuous. These inconsistencies cause undesirable channeling of fluids injected to produce oil from the tar sand. Fluid channeling causes many adverse effects including loss of efficient heat transfer.

in the prior art, mention has been made of the depth of burial of the nuclear device relative to the oil-bearing zone. The positions usually mention vary from within or near the bottom of the oil-bearing zone to some nonspecific distance below the zone. Some more specific disclosures appear to prefer that the nuclear explosive to be placed within one cavity radius of the oil-bearing zone. Presumably, this position is preferred for two reasons. First, this reduces thermal decomposition of valuable tar. Second, when the cavity collapses, the chimney would be substantially composed of only tar sand. But such aplacement would have serious drawbacks when the nuclear explosives are detonated in a standard fashion. Contrary to the teachings of the art, such broken tar sands will not remain permeable. Tar sands are incompetent, naturally deformable and adhesive especially when heated by the explosion. Voids, fractures and fissures between chunks of tar sand quickly heal.

An object of this invention is to provide a method designed particularly for tar sands which are deformable and adhesive and using multiple nuclear explosions to produce oil from such tar sands.

A further object of this invention is to provide a method of forming a relatively thin zone of high conductivity below a tar zone, or a relatively thin zone that can readily be made conductive, to permit in situ combustion, or steam or other hot fluids to be injected in a manner to transfer heat to a large volume of tar sands over a long period of time with a minimum of undesirable channeling.

A still further object of this invention is to provide a method of using larger yield nuclear explosives to assist in producing oil from tar sands and to maximize the desirable effects of the nuclear explosions while minimizing the adverse effects.

SUMMARY OF THE INVENTION in this method, the ordinary chimney geometry is modified and at the same time there is created a highly conductive, continuous, relatively thin zone below a tar sand. This thin, conductive zone is used for the injection of fluids to assist in producing oil from the tar sand. This is achieved by placing two or more nuclear explosives below a tar sand in a more competent rock different from the tar sand by a distance not to exceed 250 feet or three cavity radii. whichever is less. The explosives are spaced laterally apart and fired in a special time sequence. The explosions are timed so that the second explosion occurs in the interval between the time that a spherical cavity is formed by the first explosion and the roof of this first cavity collapses significiantly. The lateral spacing is such that some of the earth between the explosives will crater into the cavity formed by the first explosion.

When the two explosions are located and fired in this manner, the wall of the cavity formed by the first explosion provides a free surface. The second explosion craters this free surface into the first cavity in a manner similar to the way that a nuclear explosion at a shallow depth craters at the ground surface. Since the explosives are placed below the tar sand in a more competent rock and the second explosion craters or spalls rock into the first cavity, the first cavity is at least par tially filled by broken competent rock brought in from the side of the cavity rather than from collapse of the cavity roof. The

first result is that there is much less vertical growth of the first cavity. The second result is that the cavity formed by the second shot is smaller than normal since the expanding gases of the second explosion are vented into the first cavity before cavity expansion of the second shot is complete. This lowers the gas pressure in the second cavity resulting in a smaller cavity radius. The third result is that cratering of the rock between the two cavities forms an unstable horizontally extended, intermediate volume which in turn collapses or remains broken and highly fractured. Since the explosives are placed below the tar sand by a distance not to exceed 250 feet or three cavity radii, a fourth result is that the horizontally extended highly fractured volume is comprised of material other than tar sand and remains highly conductive or is readily made conductive. The overall result is to essentially form a thin, continuous, horizontal chimney of competent, conductive material below a self-healing tar sand and permit the use of larger explosives than could otherwise be used at such depths. These results are enhanced by the use of an array of first and second explosives wherein the second explosives crater laterally into more than one cavity, and wherein more than one second explosion craters into a single first cavity.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a cross section of the earth illustrating placement of three nuclear explosives below a tar sand.

FIG. 2 is a cross section of the earth illustrating the spalling or cratering effect ofa second nuclear explosive at the walls of two nuclear created cavities.

FIG. 3 is a cross section of the earth illustrating a horizontal, conductive zone below a tar sand created by three nuclear explosives.

FIGS. 4 and 5 illustrate an array of first and second nuclear explosives and the laterally extended conductive zones that could be formed below a tar sand by such an arrayl DESCRIPTION OF PREFERRED EMBODIMENTS First and second nuclear explosives are emplaced in boreholes far enough below the surface of the earth to be contained when the nuclear explosives are detonated. A nuclear explosion is contained when the explosive energy will not crater at the surface of the earth. This depth of burial permits the expanding forces to propagate along the paths hereafter described.

As will be hereinafter illustrated, the first and second nuclear explosives may each be comprised of more than one spaced apart nuclear explosive and the terms first and second are used to designate the order of firing.

In this invention, the nuclear explosives must be placed below a tar sand as this invention is applicable only to tar sands. For purposes of this invention, a tar sand is a subsurface zone or series of strata containing tar or highly viscous oil. Such sands are usually incompetent and contain a bituminous material which at subsurface reservoir conditions or at temperatures around 120 F. to 200 F. are thick, adhesive and sticky and will change form under the influence of a deforming force. Such sands are called self-healing because voids, fractures or fissures in the tar sand or between chunks of tar sand quickly seal and close.

The distance that the nuclear explosives are placed below the tar sand should not exceed a predetermined distance (x). This distance should not exceed 250 feet and should be less than three cavity radii (3R) as hereinafter defined, that is, this distance will be less than 250 feet or 3R feet whichever is less.

In every embodiment, first and second nuclear explosives are spaced laterally from each other by a predetermined lateral spacing (D). This lateral spacing distance is such that when the first nuclear explosive is detonated and the second nuclear explosive is thereafter detonated, some of the earth between the two nuclear explosions will crater into the cavity formed by the first nuclear explosive. When an array of first and second nuclear explosives are used, the explosives can be spaced in a manner such that a single second explosion can crater into more than one cavity, or in a manner such that more than one second explosion can crater into the same single cavity.

A cavity is a large space unfilled with significant amounts of broken solid material and provides a free surface which the expanding gases of a nuclear explosion may crater or spall. An explosion craters a surface of a cavity when energy from the explosion causes rock located between the explosion and the surface of the cavity to spall or blow into the cavity. Best results are obtained by increasing the lateral spacing of the two explosives provided that the second explosion is still capable of cratering large amounts of rock into the cavity created by the first explosion. This lateral spacing will be between two and IS times the cavity radius for the second nuclear explosive when the cavity radius in feet is calculated according to equation (2) hereinafter defined. Since the free surface of the cavity is comparable to the surface of the earth, the results obtained from nuclear experiments where the explosion cratered at the surface of the earth may be used to determine more preferred values for lateral spacing distance (D) at which a nuclear explosion will crater into a laterally spaced cavity. These experiments are also useful in determining a depth of burial (h) for containment of the explosive energy. A review of the results of 60 nuclear blast experiments indicates that there is a better than percent chance that the nuclear blast will crater to a free surface if the scaled distance of separation (SDS) is about 300 feet. Scaling takes into consideration the yield of the explosive. Some nuclear explosives cratered to the surface of the earth at scaled distances as great as 650 feet; therefore, a scaled distance of separation of 650 feet is an upper limit for calculating the lateral distance of separation. When the shallow depths of tar sands are taken into consideration, for maximum assurance that the second explosion will blow substantial amounts of rock into one or more first cavities. the scaled distance of separation should be 300 feet or less.

The same test results indicate that a nuclear explosion will not crater at the surface of the earth when the scaled depth of burial is greater than about 700 feet. One nuclear blast did not crater at a scaled depth of burial of 350 feet; therefore, a minimum scaled depth of burial could be set at about 350 feet. Dynamic venting at the surface of the earth will usually be prevented if there is at least a 300-foot buffer zone between the surface and the top of the chimney. The chimney height for nuclear explosions detonated in a standard manner is between four and five times the cavity radius. In this method, there is less chance that the second explosion will crater to the surface since each of the second explosives is separated from one or more first cavities by a distance of separation that will be substantially less than the depth of burial and the second explosion craters into one or more first cavities.

The scaled distance of separation (SDS) in feet between first and second nuclear explosives is converted to the actual distance of separation (D) in feet by the following equation (I):

D=(SDS) W'-" (l) where W is the yield of the nuclear device in kilotons. The scaled depth of burial is similarly related to the actual depth of burial.

Although the optimum yield for the nuclear devices will largely depend on how the formation surrounding the nuclear explosion is to be exploited, it can be said that the size of each first explosive should be large enough to provide an unstable cavity having a surface area large enough to accomplish the purposes hereof, that is, each first explosive provides a surface for one or more second explosives to crater. A cavity is unstable when the roof of the cavity will collapse after the energy for expanding the cavity has been dissipated. The first explo sive also provides an unstable cavity large enough to receive large amounts of broken rock which according to this invention will spall or be blown into the first cavity by one or more second explosions. Each second explosion has a yield which provides sufficient energy to crater rock laterally into one or more first cavities while producing a second'unstable cavity. The first and second nuclear explosives may be of different yields.

The radius of the cavity produced by each explosive is a function of energy .yield of the explosive and, to a lesser extent, the depth of burial, the average density of the overlying formations, the vapor forming liquid content of the host rock, and the rock properties. The equation for the cavity radius is where R is the cavity radius in feet, C is constant depending upon rock and fluid content and ranges between 225 and 345, W is the expected yield of the particular nuclear device in equivalent kilotons of TNT, d is the average overburden density in grams per cubic centimeter ranging from about 1.6 to 2.7, and h is the depth of burial in feet.

The method of this invention is applied to tar sands. A suitable estimate of the results for two explosives at different depths of burial and yields can be derived by using a value of 290 for the constant C, 2.2 for d and a scaled distance of separation of 300 feet in equations (1) and (2). illustrative results for two nuclear explosives of the same yield and at essentially the same depth of burial are shown in table I.

TABLE I First Scaled cavity Distance of Depth of depth of Yield, radius separation Ratio, burial, ft. burial, ft. kilotons (O R), ft. S) ft. D S/C R In table I, the ratio of the distance of separation (DS) to the cavity radius (CR) gives approximate lateral spacing between shots in terms of the cavity radius thereby illustrating the advantages of this system of firing nuclear explosions. It should be noted that this ratio is based on a scaled distance of separa tion set at 300 feet in all cases to assure cratering between cavities. Actually the distance from shot point to shot point could be increased since the cavity radius of the first cavity could be added to cratering distance. if this is done, each ratio in the column entitled Ratio, DS/Cr would be increased one unit. This ratio shows that a large amount of rock is broken laterally to produce a horizontal volume of broken rock. Since in this invention the nuclear explosives are placed in a different type of formation below a tar sand by a distance of 250 feet or less and not to exceed 3R feet, this horizontal volume of broken or fractured rock forms a thin continuous, highly conductive zone to assist in producing oil from the tar sand.

As mentioned previously, each second nuclear explosion could be arranged to crater into more than one cavity, or two or more second nuclear explosives could crater into a single cavity, or both occurrences could take place. The scaled distance of separation (SDS) and the lateral spacing (D) will be adjusted to fit the arrangement of nuclear explosives.

After the nuclear devices have been emplaced and readied for firing, the first nuclear explosive or explosives are detonated to create one or more first cavities. The energy of each explosion is generated in microseconds. The heat and pressure proceed radially outward from the shot point vaporizing, melting and crushing the enveloping medium forming an expanding cavity. Since the explosion is contained and restricted by resisting forces, the energy tends to expand the cavity uniformly in all directions with a slightly greater radius in the top half of the cavity. The shock wave from the first explosion or explosions will reach the nearest second explosive in less than 0.05 second; therefore, since in this method each second explosive is detonated after this shock wave reaches each second explosive, it is necessary to prevent mechanical damage to each second explosive so that each second device will survive the severe acceleration of thousands of gravities caused by each shock wave. This is not considered a severe problem since nuclear explosives have been designed to fire from artillery, where they also receive acceleration of thousands of gravities. For example, shock suspension cradles allowing each second device to move a foot or more would reduce peak acceleration to safe limits and each second device could be weighted to further reduce acceleration. On an average, in less than 500 milliseconds, each first cavity will reach its final size. It usually takes from 0.1 to 0.5 ofa second for a cavity to reach its final size. For example, in the Rainer experiment, the cavity reached its final radius of 65 feet in milliseconds. As mentioned previously, the yield of each first explosion is such that each first cavity is unstable, that is, the roof of each cavity would fall into the cavity under normal conditions. in this method, however, as will hereafter be shown, each second nuclear device is detonated before a significant portion of the roof of each first cavity collapses. Collapse is significant when the amount of rock falling into the cavity is greater than 10 volume percent of the cavity.

in prior nuclear tests, the time interval between when a nuclear device is detonated and the cavity starts to collapse has varied. The cavity normally stands for at least three seconds to three minutes, and longer depending on rock properties and yield of the explosion. in addition, the roof collapses by bits and pieces; consequently, it takes a measurable time to build up a significant amount of broken rock in the cavity.

After a cavity is formed by a first nuclear explosion and before the first cavity collapses, each second nuclear explosion is detonated. It has previously been noted that each second nuclear device is spaced laterally from one or more first cavities by a distance such that the energy from each second nuclear explosion will force rock between two explosions to crater into the side of one or more first cavities. This is possible only because each first cavity has not yet collapsed.

As in each first explosion, the instantaneous energy developed by each second explosion quickly forms an expanding spherical cavity. The shock wave from each second explosion travels much faster than each second cavity expands. When this shock wave reaches an empty spherical cavity formed by a first explosion, forces due to reflection of the shock wave at this free surface and the greater compressibility of this cavity compared to native rock cause the desired cratering or spalling into the side of the first cavity. The permeable path opened in this way from a second cavity to a first cavity allows the hot expanding gases from each second explosion to flow through the rock between two explosions, ever enlarging the path of penetration until the energy of these gases is vented into one or more first cavities.

Each second shot could be fired within 0.02 to 0.05 second of the first explosion or explosions, that is, before the shock wave from a first explosion reaches a second nuclear explosive, but this timing would cause interaction of shock waves and make it less likely that rock would crater into a first cavity. it is difficult to predict what would occur. Each first cavity usually reaches its final size within 0.1. to 0.5 second; therefore, it is best to detonate each second explosion at least 0.1 second after each first explosion and, more preferably, at least 0.5 second after each first explosion. in all cases, each second explosion could be reliably delayed for a second after each first explosion because rock falling from the roof of a first cavity falls only 16 feet in the first second. One second is less that any observed cavity collapse time in all contained nuclear blast experiments. in most instances, each second explosion could be delayed as long as three minutes. Even ifa first cavity started to collapse sooner, it would take time to build up a significant amount of broken rock in the cavity. Longer times have been encountered in unstable cavities.

The moment for detonating each second nuclear explosive could also be determined seismically at the surface of the earth after the initial seismic waves caused by the explosion have subsided. It will be recalled that the roof of an unstable cavity collapses in bits and pieces over an extended period of time. The pieces of rock falling to the bottom of the cavity generate seismic waves when striking the bottom. These seismic waves travel to the surface where the waves may detected by a seismometer. Thus, the second nuclear explosive or explosives could be detonated when these seismic waves are first detected.

After the explosions have been detonated, the roofs of the unstable cavities formed by the explosions and of the unstable path between the cavities collapse. This continues until the roof supports the overburden and until the volumes in the cavities and flow path are translated to the interstitial space between the broken rock which normally has a bulk porosity between 0.2 to 0.3. This forms a laterally extended chimney of broken rock between two shortened vertical chimneys rather than forming two separate tall vertical chimneys. Each chimney formed by a first cavity is shorter than usual since each first cavity has already been partially filled by broken rock blown in from the side. This blown in rock decreases the available volume to be translated into interstitial space between the pieces of rock settling from the roof of the cavity. The height of each chimney formed by a second cavity is reduced since the energy of the explosion was directed laterally toward one or more first cavities and vented into one or more first cavities. It is expected that the height of these second chimneys will be reduced one-third or more.

The nuclear explosives were placed below a tar sand in a competent formation or rock different from the tar sand. Each cavity formed by a nuclear explosion would, therefore, extend at least one cavity radius below the tar sand. When a second explosion caused the wall of the first cavity to spall into the cavity, the bottom of the cavity would thereby be covered with competent rock. This broken competent rock would extend to one or more second cavities depending on how many second explosions cratered into the first cavity.

Preferably, each second nuclear explosion will be placed below the tar sand by a distance such that there will be competent rock above each second cavity. The competent rock above each second cavity will fall first to the bottom of the cavity thereby forming layers of competent rock on the bottom of the cavity. The pulverizing effect and the size of each second cavity is reduced because the energy of the explosion vents into one or more first cavities. This reduction in cavity size is taken into consideration when placing each second explosive and enables the use of larger explosives with tar sands.

When the first and second cavities collapse, broken chunks of tar sand will collect above the conductive competent rock. Tar sands are composed of incompetent material and the chunks of tar sand will deform or heal cracks and voids between the chunks returning the chunks to an impermeable state. Experience has shown that it is expensive, if not impractical, to attempt to force fluids through impermeable tar sands and production from such incompetent sands is not improved by fractures unless such fractures are continuously held open by the pressure of the injected fluids. But the layers of broken competent rock formed below the tar sand by this invention are of a character such that even if the voids between the broken rock were partially filled with melted tar, the conductivity to fluid flow could readily be restored using injected fluids to produce oil from the tar sand. Any of the procedures suggested for producing tar sands could be used, for example, steam, hot water or hot gas injection, in situ combustion, cycling offluids, and the like.

The foregoing description of this method for assisting the production of oil from tar sand using multiple nuclear explosives may be better understood with reference to the drawings wherein FIG. I shows incompetent tar sands ll underlain by more competent rock 13. Three nuclear explosives are emplaced in rock 13 below the tar sand by average distance x. This distance is no greater than 250 feet and is less than 3R feet where R is the cavity radius in feet determined in accordance with equation (2). The depth of burial (h) of the three explosives below surface 15 is such that the explosives will be contained.

The three nuclear explosives are emplaced in boreholes l7, l9 and 21 which are laterally spaced by lateral distance D. For reasons described previously this lateral spacing is such the explosive in borehole 19 will cause rock 13 to crater or spall into the cavities created by the nuclear explosives in boreholes 17 and 21.

After the nuclear explosives have been properly emplaced and prepared for firing, the nuclear explosives in boreholes l7 and 21 are detonated simultaneously as first nuclear explosions, thereby creating cavities 23 and 25. Before these first cavities collapse, the nuclear explosive in borehole 19 is detonated as second nuclear explosion 27. As the cavity formed by this second nuclear explosion increases, a shock wave travels rapidly outward crossing the boundary of first cavities 23 and 25. This creates a returning shock wave and causes broken rock 13' at the walls of first cavities 23 and 25 to spall into the cavities. This event continues until eventually the energy from the second nuclear explosion is vented into the first cavities. Rock 13' between the first cavities and the second cavity formed by second explosion 27 is broken and fractured.

As illustrated in FIG. 3, when the energy of the nuclear explosives no longer supports the overburden, the cavities col- 7 lapse. The second cavity formed by second explosion 27 first fills with broken rock 13. It will be noted that in FIG. 1, for illustrative purposes only, borehole 19 was deeper than boreholes l7 and 21 so that there would be rock 13 above this second cavity. Above broken rock 13 in the cavities, chunks of tar sand 1 1 will collect, but as illustrated, the incompetent tar sand heals and returns to an impermeable state. After the formations have reached the desired state, wells, such as wells 29, 31 and 33, are completed into the competent broken rock below the tar sand and fluids are injected in any well-known manner or sequence to produce oil from tar sand 1]. For simplicity, wells 29, 31 and 33 are completed into the chimneys, but this is not a necessity. As illustrated, broken rock 13' is a broken volume of rock extending continuously between the wells and forms a relatively thin highly conductive zone of competent rock below the tar sand.

FIG. 4 illustrates an array of first nuclear explosives 35 and an array of second nuclear explosives 37. Nuclear explosives 35 are detonated simultaneously to form a group of properly laterally spaced first cavities. Therefore, before these first cavities collapse, second nuclear explosives 37 are detonated. The second nuclear explosives are arranged and properly laterally spaced from first cavities so that each second nuclear explosion craters into four first cavities; moreover, the center first cavities have more than one second nuclear explosion crater into it.

When the cavities formed by the array of explosions of FIG. 4 collapse, there remains a lattice work ofa broken volume of rock 13' as shown in FIG. 5. Oil from the tar sand overlying this lattice work may be effectively produced by any well known method involving fluid injection including in situ combustion. I

For simplicity the fractured areas which extend around each cavity are not shown.

Additional series of nuclear explosions could be fired in the same manner that the second explosion is detonated. In other words, a third explosion could be used to crater rock into a second cavity before the second cavity collapsed, and a fourth could crater into the third cavity and so on. It should also be recognized that various lateral arrays other than the one illustrated could be detonated in the manner described.

What is claimed is:

1. In a method of assisting production of oil from a tar sand wherein first and second nuclear explosives are placed below said tar sand, said first and second nuclear explosives being spaced laterally from each other by a distance such that when said first nuclear explosive is detonated and said second nuclear explosive is thereafter detonated, some of the earth between said second nuclear explosive and said first nuclear explosive will crater into the cavity formed by said first nuclear explosive, and said second nuclear explosive is detonated after said first nuclear explosive and before the cavity formed by said first nuclear explosive collapses, the improvement comprising placing said first and second nuclear explosives below said tar sand in a rock different from said tar sand by a distance of less than 250 feet and less than 3R feet where R is the cavity radius in feet and is determined in accordance with the following equation:

where C is a constant ranging between 225 and 345, W is the expected yield of said second nuclear explosion in equivalent kilotons of TNT, dis the average overburden density in grams per cubic centimeter ranging from about 1.6 to 2.7, and h is the depth of burial in feet.

2. An improved method of producing oil from a tar sand wherein a horizontally extended volume of rock different from a tar sand is formed below said tar sand, which method comprises:

a. placing first and second nuclear explosives below said tar sand in said rock by a distance of less than 250 feet and less than 3R feet where R is the cavity radius in feet and is determined in accordance with the following equation:

CW" :L fiLl. Me Where C is a constant ranging between 225 and 345, W is the expected yield of said second nuclear explosion in equivalent kilotons of TNT, d is the average overburden density in grams per cubic centimeter ranging from about 1.6 to 2.7, and h is the depth of burial in feet, said nuclear explosives having a yield such that upon detonation the explosions of said nuclear explosives will be contained, said second nuclear explosive being spaced laterally from said first nuclear explosive by a predetermined distance, said predetermined distance being such that when said first nuclear explosive is detonated and said second nuclear explosive is thereafter detonated, some of said rock between said second nuclear explosive and said first nuclear explosive will crater into the cavity formed by said first nuclear explosive,

b. detonating said first nuclear explosive to create a first nuclear explosion and a first cavity,

c. detonating said second nuclear explosive to create a second nuclear explosion a predetermined time after said first nuclear explosion, said predetermined time being such that said second nuclear explosion occurs after said first cavity is formed by said first nuclear explosion and before said first cavity collapses, thereby causing some of said rock below said tar sand and between said second nuclear explosive and said first cavity to be broken and some of said rock to crater into said first cavity and thereby creating a horizontally extended broken volume ofsaid rock below said tar sand, and

d. injecting fluid into said broken volume of said rock to produce oil from said tar sand.

3. The method of claim 2 wherein the predetermined time for detonation of said second explosive is at least 0.1 second after the first explosion.

4. The method of claim 3 wherein the predetermined distance that said two nuclear explosives are laterally spaced is a scaled distance of separation of 300 feet or less.

5. The method of claim 3 wherein the predetermined distance that said two nuclear explosives are laterally spaced is between 2 and times the cavity radius for said second nuclear explosive.

6. The method of claim 2 wherein the predetermined time for detonation of the second explosive is between 0.5 second and 3 minutes after the first explosion.

7. The method of claim 6 wherein the predetermined distance that said two nuclear explosives are laterally spaced is a scaled distance of separation of 3001 feet or less.

8. The method of claim 6 wherein the predetermined distance that said two nuclear explosives are laterally spaced is between 2 and 15 times the cavity radius for said second nuclear explosive.

9. An improved method of producing oil from a tar sand wherein a horizontally extended broken volume of rock different from a tar sand is formed below said tar sand, which method comprises:

a. placing two or more first nuclear explosives below said tar sand in said rock by a distance of less than 250 feet and less than 3R feet where R is the cavity radius in feet for each nuclear explosive of said first nuclear explosives and is determined in accordance with the following equation:

where C is a constant ranging between 225 and 345, W is the expected yield of second nuclear explosion in equivalent kilotons of TNT, d is the average overburden density in grams per cubic centimeter ranging from about 1.6 to 2.7, and h is the depth of burial in feet,

b. placing at least one second nuclear explosive below said tar sand in said rock by a distance of less than 250 feet and less than 3R feet where R is the cavity radius in feet and is determined in accordance with said equation, said first and said second nuclear explosives having a yield such that upon detonation the explosions of said nuclear explosives will be contained, said second nuclear explosive being spaced laterally from each of said first nuclear explosives and being spaced from at least two of said first nuclear explosives by a distance such that when said first nuclear explosives are detonated and said second nuclear explosive is thereafter detonated, some of said rock between said second nuclear explosive and said. at least two first nuclear explosives will crater into the cavities formed by said at least two first nuclear explosives,

c. detonating said at least two first nuclear explosives to create first nuclear explosions and first cavities,

d. detonating said second nuclear explosive to create a second nuclear explosion a predetermined time after said at least two first nuclear explosions, said predetermined time being such that said second nuclear explosion occurs after said first cavities are formed by said at least two first nuclear explosionsand before said first cavities collapse, thereby causing some ofsaid rock below said tar sand and between said second nuclear explosive and said first cavities to be broken and some of said rock to crater into said first cavities and thereby creating a horizontally extended broken volume of said rock below said tar sand, and

e. injecting fluid into said broken volume of said rock to produce oil from said tar sand.

110. The method of claim 9 wherein the predetermined time for detonation of said second explosive is at least 0.1 second after the first explosion.

1111. The method of claim 10 wherein the predetermined distance that said two nuclear explosives are laterally spaced is a scaled distance of separation of 300 feet or less.

112. The method of claim 10 where-in the predetermined distance that said two nuclear explosives are laterally spaced is between 2 and 15 times the cavity radius for said second nuclear explosive.

13. The method of claim 9 wherein the predetermined time for detonation of the second explosive is between 0.5 second and 3 minutes after the first explosion.

1 1. The method of claim 13 wherein the predetermined distance that said two nuclear explosives are laterally spaced is a scaled distance of separation of 300 feet or less.

15. The method of claim 13 wherein the predeten'nined distance that said two nuclear explosives are laterally spaced is between 2 and 15 times the cavity radius for said second nuclear explosive.

where C is a constant ranging between 225 and 345, W is the expected yield of said second nuclear explosion in equivalent kilotons of TNT, 11 is the average overburden density in grams per cubic centimeter ranging from about 1.6 to 2.7, and h is the depth of burial in feet,

b. placing at least two second nuclear explosives below said tar sand in said rock by a distance of less than 250 feet and less than 3R feet where R is the cavity radius for each of said at least two second nuclear explosives and is determined in accordance with said equation, said first and said second nuclear explosives having a yield such that upon detonation the explosions of said nuclear explosives will be contained, said at least two second nuclear explosives being spaced laterally from said first nuclear explosive by a distance such that when said first nuclear explosive is detonated and said at least two second nuclear explosives are thereafter detonated, some of said rock between said at least two second nuclear explosives and said first nuclear explosive will crater into the cavity formed by said first nuclear explosive,

c. detonating said first nuclear explosive to create a first nuclear explosion and a first cavity,

d. detonating said at least two second nuclear explosives to create second nuclear explosions a predetermined time after said first nuclear explosion, said predetermined time being such that said at least two second nuclear explosions occur after said first cavity is formed by said first nuclear explosion and before said first cavity collapses, thereby causing some of said rock below said tar sand and between said at least two second nuclear explosives and said first cavity to be broken and some of said rock to crater into said first cavity and thereby creating a horizontally extended broken volume of said rock below said tar sand, and

d. injecting fluid into said broken volume of said rock to produce oil from said tar sand.

17. The method of claim 16 wherein the predetermined time for detonation of said second explosive is at least 0.l second after the first explosion.

18. The method of claim 17 wherein the predetermined distance that said two nuclear explosives are laterally spaced is a scaled distance of separation of 300 feet or less.

19. The method of claim 17 wherein the predeten'nined distance that said two nuclear explosives are laterally spaced is between 2 and 15 times the cavity radius for said second nuclear explosive.

20. The method of claim 16 wherein the predetermined time for detonation of the second explosive is between 0.5 second and 3 minutes after the first explosion.

21. The method of claim 20 wherein the predetermined distance that said two nuclear explosives are laterally spaced is a scaled distance of separation of 300 feet or less.

22. The method of claim 20 wherein the predetermined distance that said two nuclear explosives are laterally spaced is between 2 and 15 times the cavity radius for said second nuclear explosive. 

1. In a method of assisting production of oil from a tar sand wherein first and second nuclear explosives are placed below said tar sand, said first and second nuclear explosives being spaced laterally from each other by a distance such that when said first nuclear explosive is detonated and said second nuclear explosive is thereafter detonated, some of the earth between said second nuclear explosive and said first nuclear explosive will crater into the cavity formed by said first nuclear explosive, and said second nuclear explosive is detonated after said first nuclear explosive and before the cavity formed by said first nuclear explosive collapses, the improvement comprising placing said first and second nuclear explosives below said tar sand in a rock different from said tar sand by a distance of less than 250 feet and less than 3R feet where R is the cavity radius in feet and is determined in accordance with the following equation: where C is a constant ranging between 225 and 345, W is the expected yield of said second nuclear explosion in equivalent kilotons of TNT, d is the average overburden density in grams per cubic centimeter ranging from about 1.6 to 2.7, and h is the depth of burial in feet.
 2. An improved method of producing oil from a tar sand wherein a horizontally extended volume of rock different from a tar sand is formed below said tar sand, which method comprises: a. placing first and second nuclear explosives below said tar sand in said rock by a distance of leSs than 250 feet and less than 3R feet where R is the cavity radius in feet and is determined in accordance with the following equation: where C is a constant ranging between 225 and 345, W is the expected yield of said second nuclear explosion in equivalent kilotons of TNT, d is the average overburden density in grams per cubic centimeter ranging from about 1.6 to 2.7, and h is the depth of burial in feet, said nuclear explosives having a yield such that upon detonation the explosions of said nuclear explosives will be contained, said second nuclear explosive being spaced laterally from said first nuclear explosive by a predetermined distance, said predetermined distance being such that when said first nuclear explosive is detonated and said second nuclear explosive is thereafter detonated, some of said rock between said second nuclear explosive and said first nuclear explosive will crater into the cavity formed by said first nuclear explosive, b. detonating said first nuclear explosive to create a first nuclear explosion and a first cavity, c. detonating said second nuclear explosive to create a second nuclear explosion a predetermined time after said first nuclear explosion, said predetermined time being such that said second nuclear explosion occurs after said first cavity is formed by said first nuclear explosion and before said first cavity collapses, thereby causing some of said rock below said tar sand and between said second nuclear explosive and said first cavity to be broken and some of said rock to crater into said first cavity and thereby creating a horizontally extended broken volume of said rock below said tar sand, and d. injecting fluid into said broken volume of said rock to produce oil from said tar sand.
 3. The method of claim 2 wherein the predetermined time for detonation of said second explosive is at least 0.1 second after the first explosion.
 4. The method of claim 3 wherein the predetermined distance that said two nuclear explosives are laterally spaced is a scaled distance of separation of 300 feet or less.
 5. The method of claim 3 wherein the predetermined distance that said two nuclear explosives are laterally spaced is between 2 and 15 times the cavity radius for said second nuclear explosive.
 6. The method of claim 2 wherein the predetermined time for detonation of the second explosive is between 0.5 second and 3 minutes after the first explosion.
 7. The method of claim 6 wherein the predetermined distance that said two nuclear explosives are laterally spaced is a scaled distance of separation of 300 feet or less.
 8. The method of claim 6 wherein the predetermined distance that said two nuclear explosives are laterally spaced is between 2 and 15 times the cavity radius for said second nuclear explosive.
 9. An improved method of producing oil from a tar sand wherein a horizontally extended broken volume of rock different from a tar sand is formed below said tar sand, which method comprises: a. placing two or more first nuclear explosives below said tar sand in said rock by a distance of less than 250 feet and less than 3R feet where R is the cavity radius in feet for each nuclear explosive of said first nuclear explosives and is determined in accordance with the following equation: where C is a constant ranging between 225 and 345, W is the expected yield of second nuclear explosion in equivalent kilotons of TNT, d is the average overburden density in grams per cubic centimeter ranging from about 1.6 to 2.7, and h is the depth of burial in feet, b. placing at least one second nuclear explosive below said tar sand in said rock by a distance of less than 250 feet and less than 3R feet where R is the cavity radius in feet and is determined in accoRdance with said equation, said first and said second nuclear explosives having a yield such that upon detonation the explosions of said nuclear explosives will be contained, said second nuclear explosive being spaced laterally from each of said first nuclear explosives and being spaced from at least two of said first nuclear explosives by a distance such that when said first nuclear explosives are detonated and said second nuclear explosive is thereafter detonated, some of said rock between said second nuclear explosive and said at least two first nuclear explosives will crater into the cavities formed by said at least two first nuclear explosives, c. detonating said at least two first nuclear explosives to create first nuclear explosions and first cavities, d. detonating said second nuclear explosive to create a second nuclear explosion a predetermined time after said at least two first nuclear explosions, said predetermined time being such that said second nuclear explosion occurs after said first cavities are formed by said at least two first nuclear explosions and before said first cavities collapse, thereby causing some of said rock below said tar sand and between said second nuclear explosive and said first cavities to be broken and some of said rock to crater into said first cavities and thereby creating a horizontally extended broken volume of said rock below said tar sand, and e. injecting fluid into said broken volume of said rock to produce oil from said tar sand.
 10. The method of claim 9 wherein the predetermined time for detonation of said second explosive is at least 0.1 second after the first explosion.
 11. The method of claim 10 wherein the predetermined distance that said two nuclear explosives are laterally spaced is a scaled distance of separation of 300 feet or less.
 12. The method of claim 10 wherein the predetermined distance that said two nuclear explosives are laterally spaced is between 2 and 15 times the cavity radius for said second nuclear explosive.
 13. The method of claim 9 wherein the predetermined time for detonation of the second explosive is between 0.5 second and 3 minutes after the first explosion.
 14. The method of claim 13 wherein the predetermined distance that said two nuclear explosives are laterally spaced is a scaled distance of separation of 300 feet or less.
 15. The method of claim 13 wherein the predetermined distance that said two nuclear explosives are laterally spaced is between 2 and 15 times the cavity radius for said second nuclear explosive.
 16. An improved method of producing oil from a tar sand wherein a horizontally extended broken volume of rock different from a tar sand is formed below said tar sand, which method comprises: a. placing at least one first nuclear explosive below said tar sand in said rock by a distance of less than 250 feet and less than 3R feet where R is the cavity radius in feet and is determined in accordance with the following equation: where C is a constant ranging between 225 and 345, W is the expected yield of said second nuclear explosion in equivalent kilotons of TNT, d is the average overburden density in grams per cubic centimeter ranging from about 1.6 to 2.7, and h is the depth of burial in feet, b. placing at least two second nuclear explosives below said tar sand in said rock by a distance of less than 250 feet and less than 3R feet where R is the cavity radius for each of said at least two second nuclear explosives and is determined in accordance with said equation, said first and said second nuclear explosives having a yield such that upon detonation the explosions of said nuclear explosives will be contained, said at least two second nuclear explosives being spaced laterally from said first nuclear explosive by a distance such that when said first nuclear explosive is Detonated and said at least two second nuclear explosives are thereafter detonated, some of said rock between said at least two second nuclear explosives and said first nuclear explosive will crater into the cavity formed by said first nuclear explosive, c. detonating said first nuclear explosive to create a first nuclear explosion and a first cavity, d. detonating said at least two second nuclear explosives to create second nuclear explosions a predetermined time after said first nuclear explosion, said predetermined time being such that said at least two second nuclear explosions occur after said first cavity is formed by said first nuclear explosion and before said first cavity collapses, thereby causing some of said rock below said tar sand and between said at least two second nuclear explosives and said first cavity to be broken and some of said rock to crater into said first cavity and thereby creating a horizontally extended broken volume of said rock below said tar sand, and d. injecting fluid into said broken volume of said rock to produce oil from said tar sand.
 17. The method of claim 16 wherein the predetermined time for detonation of said second explosive is at least 0.1 second after the first explosion.
 18. The method of claim 17 wherein the predetermined distance that said two nuclear explosives are laterally spaced is a scaled distance of separation of 300 feet or less.
 19. The method of claim 17 wherein the predetermined distance that said two nuclear explosives are laterally spaced is between 2 and 15 times the cavity radius for said second nuclear explosive.
 20. The method of claim 16 wherein the predetermined time for detonation of the second explosive is between 0.5 second and 3 minutes after the first explosion.
 21. The method of claim 20 wherein the predetermined distance that said two nuclear explosives are laterally spaced is a scaled distance of separation of 300 feet or less.
 22. The method of claim 20 wherein the predetermined distance that said two nuclear explosives are laterally spaced is between 2 and 15 times the cavity radius for said second nuclear explosive. 